Set up the cron job with your cpanel, it runs behind the scenes and takes very little time and resources to generate your sitemap.I have over 2 million posts and it runs less than 30 minutes.

Also the guests member group, if they cannot see the urls your are missing then the sitemap cannot see it either.

More advice turn off your calendar from guests and search bots seeing it or you will generate useless pages of sitemaps with Calendar url's and the sitemap generator will not honor the Calendar dates restricted to in the admin cp.